Microcantilever sensors are known to be excellent chemical and biological sensors. The Gibbs free energy of a surface is decreased by adsorption, and when molecular adsorption is confined to one surface of a microcantilever, this leads to differential surface forces between the two sides of the cantilever. When the spring constant of a microcantilever is of the same magnitude as the free energy change due to surface adsorption on it, the microcantilever undergoes deflection due to the adsorption-induced stress.
Microcantilever-based sensors have been shown to be extremely sensitive. However, microcantilevers coated on one surface with an electrical conductor (e.g. gold) do not provide chemical selectivity. A microcantilever coated on one surface with gold has achieved chemical selectivity by adsorbing a selection film on the gold coated side of the microcantilevers, such as a self-assembled monolayer (SAM) of an alkane thiol having a head group suitable for molecular recognition. Selective coatings have been developed for sensing a variety of different ions or other chemical species.
As an alternative to selective coatings, controlled potential electrochemical techniques provide a comparatively simple method for achieving chemical selectivity by controlling the potentials at which oxidation and reduction reactions occur. When using controlled potential electrochemical techniques, chemical selectivity is conventionally achieved using a cantilever coated on one surface with a metallic conductor as the working electrode.
The model of an electrified interface between an electrolyte solution and a conductor is well developed. Graham derived much of the understanding from seminal measurements of interfacial tension at an aqueous solution-mercury electrode interface. Investigations of surface stress during electrochemical processes have been reported for macro cantilevers and microcantilevers, but a rigorous description of the charge and potential dependence of the surface tension remains a challenge. These studies have demonstrated that the dependence of surface stress on charge and potential at the solid-electrolyte interface offer more insight into the processes occurring at an electrified interface in solution during electron transfer reactions and ionic adsorption processes.
Surface charge density and surface energy are related. Thermodynamics also provides a relationship between surface free energy, surface coverage, and surface stress. This relation can be differentiated with respect to potential to obtain the generalized Lippman equation that allows the derivative of surface stress with respect to potential to be related to the surface charge density during an electrochemical process.
In summary, the deflection of a microcantilever is proportional to the surface stress that is also related to the free energy change. Thus, the derivative of the surface stress with respect to electrical potential can be related to the surface charge density.
The differences between a cyclic voltammogram (current vs. potential) and surface stress variation with potential may provide insights into the physical and chemical processes, which accompany redox reactions at modified electrodes and additional information about changes in energetics at the solid-electrolyte interface. Such information could help in understanding double layers and diffusion layers in electrolyte solutions as well as changes which accompany charge transfer at the interface. The charge transfer effects are known to have a pronounced influence on the adsorbate-induced surface stress.
Although electrochemical cantilever-based sensors are presently useful for certain applications, the use of a microcantilever as the working electrode to date has revealed several significant limitations, including the following:                1) During prototyping of an electrochemical microcantilever sensor, it is generally necessary to carry out simultaneous measurement of electrochemical current and cantilever deflection so that the electrochemical reactions and other conditions of analysis are well defined.        2) Small spring constant cantilevers are required for high sensitivity. Accordingly, the surface area of a high sensitivity cantilever is kept extremely small, such as 100 μm by 10 μm, as compared to the much larger area of the base chip, and it is difficult to limit the area which is exposed to an analyte solution to the cantilever alone.        3) The current density that can be supported on a microcantilever is small due to the small surface area of the cantilever and measurement of the small current at an electrode with this surface area requires expensive amplifiers and special shielding to eliminate background electromagnetic interference.        4) When the cantilever is functioning as a working electrode its efficiency can decrease due to irreversible reactions that occur during use. Partial coverage of the cantilever surface by contaminants leads to poor cantilever performance due to a further decrease in the effective working surface area of the cantilever.        5) The materials that can be used on a cantilever as a working electrode are limited to those metals which can be deposited by an evaporation process at a temperature low enough to prevent damage to the cantilever itself, form an adherent layer on the cantilever, and at the same time are materials that are appropriate for electrochemical reactions in an aqueous environment. The materials that can be used as a working electrode are further limited since most cantilevers are fabricated out of silicon and coated with thin layers of specific metals. Coating cantilevers with metals brings in a variety of problem, including problems due to adhesion, crystalline nature, diffusion of other metals that are used as adhesion layers through the grain boundaries, and contamination. The materials that can be plated on one surface of a cantilever are thus generally limited to platinum and gold.        
What is needed is an electrochemical cantilever-based sensor system that provides high sensitivity and overcomes the significant limitations of conventional electrochemical cantilever-based sensors noted above. It is also desirable to have electrochemical sensors that can be used in microfluidic applications in which a small surface area electrode is dictated by the confined geometry of the channels required by the application. In this case a small surface area electrode necessarily leads to small current levels. The measurement of small currents requires that the electrode be carefully shielded to prevent interference from electromagnetic radiation. This leads to a situation in which the advantages of small size brought by microfluidic devices, such as a lab-on-a-chip devices, are lost due to required electromagnetic shielding requirements which are incompatible with MEMS.